The present invention relates to compositions having particular utility in the production of film. More particularly, the present invention relates to compositions comprising a blend of a linear ultra low density polyethylene (LULDPE) and a propylene polymer, having particular utility in the production of film, especially stretch wrap cling film.
Blown film is preferable over cast film where high strength films are required. Linear low density polyethylene (LLDPE) and linear ultra low density polyethylene (LULDPE) are the most suitable for blown stretch film. However, for commercial applications, in the stretch wrapping industry, neither blown LLDPE nor blown LULDPE film has enough peel cling strength when stretched to 200 percent elongation, the commercially acceptable amount of stretching commonly required in stretch wrapping processes. Propylene polymer cling additives have been blended with LLDPE to provide monolayer blown stretch wrap film having commercially adequate cling at 200 percent elongation. The term xe2x80x9ccommercially adequate clingxe2x80x9d is hereinafter defined as a peel cling strength of at least 100 g/2.54 cm, for convenience and clarity.
Multilayer blown stretch film is more versatile than monolayer film for many purposes. In an A/B/A or A/B/C type multilayer film, the core layer is typically about 80 weight percent of the film, with each of the two skin layers making up about 10 weight percent of the film.
Blown LLDPE and LULDPE have been found to be inadequate skin layers in multilayer stretch wrap film. They do not provide enough peel cling strength when the multilayer film is stretched to 200 percent elongation. Propylene polymer cling additives have been blended with LLDPE for use in multilayer blown stretch film. However, LLDPE skin layers require a relatively high concentration of propylene polymer cling additive to be effective.
In light of the above, it would be desirable to provide a new blown film composition suitable for use in making both monolayer blown stretch cling film and multilayer blown stretch cling film. It would be further desirable if a stretched multilayer film comprising a skin layer formed from such new composition would provide adequate cling.
The present invention relates to a composition of matter which comprises a blend of about 80 to about 99 weight percent based on the composition of a linear ultra low density polyethylene having a density of less than about 0.916 g/cm3, and about 1 to about 20 weight percent based on the composition of a propylene polymer comprising 0 to about 40 weight percent based on the copolymer of olefin comonomer units having 2 to 10 carbon atoms and from about 60 to about 100 weight percent propylene, based on the copolymer, said propylene polymer having a Brookfield Thermosel viscosity of about 1 to about 30,000 mPaxc2x7s at 190xc2x0 C. and a needle penetration hardness of about 5 to about 300 dmm at 23xc2x0 C.
The applicant was surprised to find that particular propylene polymers can be usefully blended with LULDPE to form a composition useful in making both monolayer and multilayer blown stretch cling film and cast film. Film formed from the LULDPE composition of the present invention provides unexpectedly effective cling when used as a skin layer of a multilayer stretch cling film.
The composition of the present invention comprises about 80 to about 99 weight percent LULDPE component based on the composition, and about 1 to about 20 weight percent propylene polymer based upon the total weight of the composition. More particularly, the present composition comprises LULDPE having a density of less than about 0.916 g/cm3, and about 1 to about 20 weight percent propylene polymer containing 0 to about 40 weight percent olefin comonomer units having 2 to 10 carbon atoms. The propylene polymer has a Brookfield Thermosel viscosity of about 1 to about 30,000 mPaxc2x7s at 190xc2x0 C., and a needle penetration of about 5 to about 300 dmm at 23xc2x0 C.
LULDPE has excellent direction (MD) elongation, good tear resistance, good tensile strength, and good puncture resistance. LULDPE is a copolymer of ethylene and an alpha olefin comonomer, with density of less than about 0.916 g/cm3, preferably from about 0.88 to about 0.916 g/cm3, more preferably about 0.9 to about 0.914 g/cm3. LULDPE has a melt index (MI) of about 0.4 to about 8 g/10 min., preferably about 0.5 to about 4 g/10 min. The LULDPE used is more preferably an ethylene-octene or ethylene-hexene LULDPE copolymer. A minor amount of other film-forming polyolefins such as LDPE, LLDPE, polypropylene, and ethylene vinyl acetate copolymer (EVA) may also be present in the film-forming component. The present composition comprises preferably about 85 to about 99 weight percent LULDPE, and from about 1 to about 15 weight percent propylene polymer, based on the composition.
The alpha olefin comonomer of the ethylene alpha olefin copolymer preferably has from 3 to 20 carbon atoms per molecule and is present in an amount of from about 2 to about 20 weight percent based on the ethylene alpha olefin copolymer. Examples of the alpha olefin comonomer include 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, a terpolymer of ethylene, a terpolymer of 1-butene, a terpolymer of 1-hexene, and mixtures and derivatives thereof. In a preferred embodiment, the alpha olefin is selected from 1-butene, 1-hexene and 1-octene.
The LULDPE component of the composition may be prepared readily by any conventional technique known in the art.
The present propylene polymer comprises 0 to about 40 weight percent C2 to C10 olefin comonomer unit content and from about 60 to about 100 weight percent propylene, based upon the total propylene polymer. The propylene polymer preferably comprises 0 to about 25 weight percent comonomer unit content, and from about 75 to about 100 weight percent propylene, based upon the polymer, more preferably 0 to about 20 weight percent comonomer unit content and from about 80 to about 100 weight percent propylene. The olefin comonomer units are preferably ethylene units.
A particularly surprising aspect of the present invention is the broad range of molecular weights and hardness levels possible for a suitable propylene polymer. The molecular weight is evidenced by the viscosity at 190xc2x0 C., determined by the Brookfield Thermosel viscosity method. The viscosity range of the propylene polymer of the present invention is from about 1 to about 30,000 mPaxc2x7s, at 190xc2x0 C. The viscosity is preferably from about 1 to about 15,000 mPaxc2x7s, at 190xc2x0 C., more preferably from about 1 to about 7,000 mPaxc2x7s, at 190xc2x0 C. with about 1 to about 5,000 mPaxc2x7s, at 190xc2x0 C. being most preferred. A composition comprising a propylene polymer having a viscosity of from about 1 to about 2,000 mPaxc2x7s at 190xc2x0 C. is most suitable for use in producing blown film.
The propylene polymer of the present invention can contain a measurable amount of crystallinity, more than 1 weight percent by a hexane extraction method such as ASTM D5227, or can be totally amorphous. The propylene polymer has a needle penetration range of about 5 to about 300 dmm, determined by ASTM (test method modified to 23xc2x0, instead of 25xc2x0 C.). The propylene polymer preferably has a needle penetration of about 5 to about 200 dmm at 23xc2x0 C., with about 5 to about 100 dmm at 23xc2x0 C. being more preferable.
The propylene polymer component of the composition utilized herein may be produced by a process disclosed in Canadian Patent 1,296,484 entitled xe2x80x9cProcess for the Direct Synthesis of Highly Amorphous Propylene Homopolymers and Propylene-Ethylene Copolymers.xe2x80x9d Some of the propylene copolymer components within the scope of the present invention may also be purchased from Eastman Chemical Company, Kingsport, Tenn. under the Eastman Chemical Company Trademark, EASTOFLEX.
The exact amounts of the LULDPE component and the propylene polymer component of the compositions herein are determined by the application for which film produced therefrom is to be employed. Variation of the amounts of each component is within the scope of one skilled in the art.
For many purposes, it may be desirable to incorporate other conventional additives with the compositions of the present invention. For example, there may be added antioxidants, heat and light stablilizers, dyes, antistatic agents, preservatives, processing aids, pigments, flame retardants, and the like.
The compositions of the LULDPE component defined hereinabove and the propylene polymer defined hereinabove are readily prepared by any method known in the art. For example, the components of the composition may be blended together by melt extrusion or may be admixed together on a conventional mixing machine such as an extruder or a continuous mixer.
The composition of the present invention has good peel cling strength for use in forming stretch wrap film. For purposes of the present application, commercially adequate peel cling strength has been determined to be at least 100 g/2.54 cm, measured at 200 percent elongation. It is now commonplace for stretch wrap film to be elongated to 200 percent and higher, during a stretch wrapping process. Peel strength generally decreases with more elongation. Therefore, it is important to determine that the peel cling strength of a film be adequate at 200 percent elongation, instead of being measured only when unstretched, as has been past practice. At 200 percent elongation, a film (both monolayer and multilayer films) formed from the composition of the present invention preferably has a peel cling strength of about 100 to about 500 g/2.54 cm, preferably abut 100 to about 400 g/2.54 cm, with about 100 to about 300 g/2.54 cm being more preferable, determined using ASTM Method D5458-95 xe2x80x9cStandard Test Method for Peel Cling of Stretch Wrap Film.xe2x80x9d
The composition of the present invention can be formed into stretch wrap film using any technique known in the art, such as a cast method or blown film method. The present invention includes a blown film process wherein an extruder having an annular die is used for extruding the composition of the present invention. Air is blown in through the extruder die to form a bubble of the present polymer blend having a blow-up ratio of 1 to 3.5 times the die diameter. After a cooling air stream cures the film, the film is wound onto rolls. More particularly, a composition, as defined hereinabove is introduced into the feed hopper of an extruder that is water-cooled and resistance heated. The films were produced using an annular die having a die gap, of about 2.24 mm having a dual orifice nonrotating, nonadjustable air ring. The film is extruded through the die into a film that is cooled by blowing air onto the surface of the film in a blowing manner. The film is drawn from the die typically forming a cylindrical film that is cooled, collapsed, optionally slit, and wound to form a roll of film.
In preparing cast film, any of the well known cast film forming procedures may be utilized. For example, a composition, as defined hereinabove, is introduced into the feed hopper of an extruder that is water cooled and resistance heated. The films were produced using a slot-type die where a sheet of the polymer is extruded onto chilled rolls. After cooling by the chilled rolls, the resulting film is wound onto film cores. Optionally, the film may be slit prior to winding the film onto the film cores.
The present invention further includes a film of the composition of the present invention. The preferable film is a blown stretch wrap cling film. The film generally has higher puncture strength and a higher gloss than the neat LULDPE. The film made from the composition of the present invention generally also has a high speed unwinding noise not greater than the unwinding noise of neat LULDPE. This is important in light of the OSHA noise workplace restrictions. The present invention also includes articles of manufacture comprising a film of the composition of the present invention. Such articles include monolayer films and multilayer films in which the film of the composition of the present invention is at least one layer.
The present invention further includes a process for stretch wrapping, using a film of the composition of the present invention. The process comprises wrapping the film of the present invention about the girth of a unitized plurality of goods so that the film is in a stretched condition and the end region of the film is attached to a previous area of film by cling force.
The invention will be more readily understood by reference to the following examples. There are, of course, many other forms of the invention which will become obvious to one skilled in the art, once the invention has been fully disclosed, and it will accordingly be recognized that these examples are given for the purpose of illustration only, and are not to be construed as limiting the scope of this invention in any way.